In recent years, there is developed a device based on the H.264/AVC (Advanced Video Coding) scheme, in which image data is handled as digital data, at that time, for the purpose of transmitting and storing information with a high efficiency, the data is compressed by a discrete cosine transform or other orthogonal transform and motion compensation, by using the redundancy peculiar to image information.
In the MPEG (Moving Picture Experts Group) 2, the number of reference frames is two, and the number of the past reference picture located before the present picture is always one.
Contrary to this, in the H.264/AVC, it is permitted to have a plurality of reference frames, at the same time, for example, as shown in FIG. 10(A), it is also possible to refer to further past pictures over an I picture.
Accordingly, even when starting decoding from the I picture, it is not guaranteed that the decoding can be correctly carried out. This becomes a big problem in a random access etc. Therefore, in the H.264/AVC, a picture called as an IDR (Instantaneous Decoder Refresh) picture as shown in FIG. 10(B) is prescribed. With H.264/AVC, when decoding an IDR picture, all information stored in the memory which are required for the decoding such as the reference frames, frame numbers, and POC (Picture Order Count: information indicating an output order of pictures) are reset. Accordingly, it is prohibited to refer to further past pictures exceeding the IDR picture. Further, with an IDR picture, a reference frame memory, a buffer, etc. are initialized. When the decoding is started from an IDR picture, correct decoding of the image is guaranteed.